The present invention relates to a method of predicting the state of charge of a rechargeable battery, in particular a rechargeable lithium battery. Moreover, the present invention relates to a use time left system, which calculates and indicates the time that an application can be used under predefined conditions.
U.S. Pat. No. 5,631,540 relates to a method and apparatus for predicting the remaining capacity and reserve time of a battery on discharge. According to said patent, the remaining battery capacity is determined from the difference between the battery full charge open circuit voltage and the voltage loss due to internal resistance of the battery minus battery voltage on discharge divided by the battery temperature, which is the temperature-corrected battery overvoltage. The remaining reserve capacity of the battery is subsequently determined by the ratio of the remaining reserve capacity calculated earlier to a maximum theoretical capacity as an exponential function.
An important disadvantage of the above method is that the calculated remaining battery capacity will eventually drift away from the real value owing to inter alia measurement inaccuracies. This results in an inaccurate prediction of the remaining capacity and reserve time of the battery.
The present invention aims to provide a method of predicting the state of charge of a rechargeable battery which provides an accurate measurement of the remaining capacity of the battery.
To this end, the present invention provides a method of predicting the state of charge of a rechargeable battery, in particular a lithium battery, comprising the steps of:
determining whether the battery is in an equilibrium state or in a non-equilibrium state; and
if the battery is in an equilibrium state, measuring the voltage across the battery and converting this measured voltage into an equilibrium state-of-charge value;
if the battery is in a non-equilibrium state, calculating the charge withdrawn from or supplied to the battery by means of current integration, and subtracting this charge from or adding it to a state-of-charge value calculated earlier.
The battery is said to be in the equilibrium state if only a very small amount of current is drawn from or supplied to the battery. The absolute value of said current is lower than a defined small current Ilim. This situation occurs, for example, when a mobile phone is in the standby mode. In such a case the current drawn from the battery is only a few mA.
The non-equilibrium state comprises the charge state, the discharge state and the transition state. In the charge state, a positive current larger than the defined Ilim is flowing into the battery. In the discharge state, a negative current larger than the defined Ilim in absolute value is flowing out of the battery. The transition state is the state in which either the charge state or discharge state changes to the equilibrium state.
A more accurate measurement of the state-of-charge is achieved in that a distinction is made between the calculation of the state of charge in the equilibrium state and in the non-equilibrium state of the battery.
The measured voltage across the battery in the equilibrium state is converted into an equilibrium state-of-charge value in particular through the use of a stored characteristic voltage versus state of charge curve, preferably the EMF curve.
As in the equilibrium state only a small amount of current flows from the battery, the measured voltage approaches the EMF of the battery. This EMF equals the sum of the equilibrium potentials of the electrodes of the battery. The method according to the present invention is based on an algorithm which uses a stored EMF versus state of charge curve to translate the voltage value measured in the equilibrium state into a state-of-charge value, expressed as an percentage of the maximum capacity.
The EMF versus state of charge curve remains the same, even when the battery ages. Moreover, the temperature dependence of this curve is relatively low. Thus the EMF curve is suitable for use as a calibration of the state-of-charge system, because the same state-of-charge is found for a certain measured EMF value, irrespective of the age and temperature of the battery. This calibration is important because in the non-equilibrium states the calculated state of charge will eventually drift away from the real value owing to, for example, inaccuracies in current measurement and the integration in time of said inaccuracies.
Since the method of estimating the state of charge comprises a calibration step in the equilibrium state, it is important that in said state the voltage actually does approach the EMF of the battery. Therefore, the algorithm is only allowed to enter the equilibrium state when a steady-state situation has been reached, in which the battery voltage is close to or substantially equals the EMF.
In the non-equilibrium state the charge withdrawn from or supplied to the battery is calculated by means of current integration, and this charge is subtracted from or added to a state-of-charge value calculated earlier. This method is also called Coulomb counting.
In particular, the state-of-charge value calculated earlier may be the initial state-of-charge value or a previous equilibrium state-of-charge value.
The algorithm according to the method of the invention operates in five states, these being the initial state, the equilibrium state, the charge state, the discharge state and the transition statexe2x80x94the latter three being the non-equilibrium states. The algorithm starts up in the initial state and determines the initial state of charge.
Advantageously, the initial state-of-charge value is obtained by measuring the voltage across the battery in the initial state and converting this measured voltage into an initial state state-of-charge value.
Dependent on whether the battery is charged, discharged or in equilibrium, the algorithm then shifts to the appropriate state and determines the state-of-charge by voltage measurement or by Coulomb counting.
The present invention also relates to a method of predicting the time that an application can be used under predefined conditions, the so-called time left. Said method comprises the steps of:
determining the state of charge of the battery according to the method of the present invention as disclosed in the above; and
calculating the battery voltage as the sum of the equilibrium voltage and the overpotential.
The voltage of a battery during discharging is lower than the equilibrium voltage. This is due to the overpotential. A distinction should be made between the available charge in the battery and the charge that can be withdrawn from the battery under certain conditions. Especially at low temperatures and at a low state of charge, the remaining charge cannot be withdrawn from the battery owing to a high overpotential, because the battery voltage will drop below the so-called end-of-discharge voltage Vmin defined in the portable device.
A battery is considered empty when the voltage drops below a certain level Vmin. For example, in mobile telephone terminology, xe2x80x9ctalk time leftxe2x80x9d refers to the time it takes until the voltage drops below Vmin while the telephone is continuously in the talk mode. xe2x80x9cStandby time leftxe2x80x9d refers to the time it takes until the voltage drops below Vmin while the telephone is continuously in the standby mode.
Advantageously, the overpotential is calculated from a function having at least a time-dependent part. Preferably, the overpotential is calculated from a function at least comprising a capacity-dependent part.
In order to predict the battery voltage, it is necessary to know the state of charge of the battery and to calculate the overpotential as a function of time and the state of charge.
In particular, the overpotential is calculated from a function having at least a temperature-dependent part.
Such a temperature dependence allows to determine the use time left under different temperature conditions, for example the use time left at room temperature as well as the use time left at zero degrees Celsius. In the calculations, the standby time-left and talk time left can also be taken into account, resulting in an indication of standby time left at two different temperatures, for example at room temperature and at a temperature of zero degrees Celsius, and an indication of talk time left at two different temperatures.
In the non-equilibrium state, the state of charge of a battery is calculated by means of Coulomb counting. The overpotentials are calculated in a differential form, as shown in equations 7 to 12 represented below. Both the overpotentials and the Coulomb counting are reset in the equilibrium state.
In particular, the overpotential is calculated from a function having at least one term which is inversely related to the remaining capacity of the battery.
Finally, the present invention relates to an electronic network model of a rechargeable battery comprising at least one RC circuit wherein the values of the resistor and the capacitor are related to a time-dependent part of the overpotential of the battery, at least two resistors, the value of one resistor being related to the remaining capacity of the battery which corresponds to the overpotential, and a voltage source whose value is related to the remaining capacity of the battery which corresponds to the equilibrium voltage.
In particular, the value of one or more of the components in the electronic network has a temperature dependence.
In order to achieve maximum accuracy, the values of all components in the electronic network are dependent on the remaining capacity of the battery.
In particular, the value of one of the resistors depends on the direction of the current. Higher accuracy may be obtained if also this resistor is connected in parallel with a capacitor in order to simulate the time dependence of this particular process.